The present invention relates to high frequency power transistors, and, more specifically, to high frequency power transistors having an improved contact structure.
High frequency power transistors typically utilize small, closely spaced emitter and base contact areas connected to bonding pads remote from the active areas of the transistor where heat is generated. Small, closely spaced emitter and base contact areas are utilized because emitter injection is largely concentrated at the periphery at high current levels rather than being uniformly distributed over the entire emitter area. Consequently, it is important to have large peripheries between emitter and base areas in order to obtain large currents and thus obtain high levels of power.
A typical microwave power transistor utilizes a relatively large area base pocket including a plurality of rows of tiny emitter sites (contact areas), such rows being closely spaced between successive base contact areas. The emitter metalization pattern typically comprises a finger electrode structure connected to a bonding pad at one side of the base pocket. The "fingers" extend across successive rows of emitter sites in the base pocket and thus "overlay" the base region between successive sites. The base metalization pattern also comprises a finger electrode structure interleaved with the first and connected to a bonding paid on the opposite side of the base pocket.
A number of electrical characteristics of the contact structure limit the frequency response, bandwidth, and power output of microwave power transistors. Such limiting characteristics include inductance and resistance in the device-to-packaging interconnection system, voltage drops in the emitter and base metal pattern contacts, and MOS capacitance between the emitter plate and base bond pads and the silicon substrate.
Heat also poses a serious problem to the performance and operating life of such transistors. Heat generated at the collector-base junction, near the surface of the transistor, can lead to a temperature buildup which can cause metal to diffuse and eventually short-circuit the emitter and base.
Because high frequency power transistors utilize small, closely-spaced emitter and base contact areas, it is particularly difficult to remove heat from the active area of the transistor. Bonding wires, which can be one path of heat removal, are generally connected to bonding pads remote from the closely spaced electrodes disposed over the active areas. Consequently, there is substantial thermal resistance between the active area and the bonding wires.
The closely spaced electrode structures of typical high frequency, high-power transistors have also effectively precluded the use of "flip chip" mounting to remove heat. In flip chip mounting, the transistor is "flipped" and mounted with its electrodes bonded to corresponding bonding pads on a heat sink and its active surface in thermal contact with the heat sink through the electrode bonding. This approach, however, is impractical with high frequency, high power transistors because the closely spaced electrode structure over the active area (1) presents substantial thermal resistance between the active area and any remote bonding points, and (2) presents insoluble alignment problems precluding direct bonding of the fine electrode structure to a corresponding fine structure on the heat sink. Moreover, even if alignment were possible, the emitter and base electrodes are so closely spaced, i.e., on the order of 0.1 mil, that conventional bonding techniques, such as soldering or ultrasonic bonding, would short the two electrode structures.
Accordingly, there is a need for an improved contact structure for microwave power transistors.